Introducing Instrumental Case
It's not a box used for instruments, it's a grammatical case! Well... actually it has something to do with tools and instruments, among other things. It's used to indicate manner of doing an action, tools used, and of course certain prepositions need it!
The instrumental case (abbreviation: ins. or I) is a very cheap case: one has to remember only endings for singular, since it's always equal to the dative in plural! Now, isn't it easier than expected? Here is the scheme for nouns:
case ma-nouns mi-nouns n-nouns a-nouns i-nouns nom.sg. - - -œ -a - acc.sg. -a -u dat.sg. -u (+)i -i gen.sg./dual -a -e -i ins.sg. -œm -om -i, -ju nom.pl. -[œv]+i -a -e -i acc.pl. -[œv]e dat./ins.pl. -[œv]+ima -ima -ama -ima gen.pl. -[œv]a -a -a -i
Recall that symbol œ means 'o or e, depending on the previous sound(s), according to the e/o rule'. The rule is always the same, I've introduced it in 10 M-Nouns and the o/e Rule.
There's a small complication: some i-nouns can also have -ju in instrumental sg., instead of -i. If an i-noun ends on -t in nom., it 'fuses' with -ju to make -ću; also sometimes lj is found instead of l; other assimilations are possible as well. (If you are interested in the precise definition of this sound change, check 34 Degrees of Adjectives, J-Softening.) For instance:
krv "blood" → krvi or krvlju
mast "fat" → masti or mašću
sol "salt" → soli or solju
However, many i-nouns like noć, kost etc. have only the -i form of the ins. sg. It's perfectly OK to use only the -i forms for all i-nouns!
Now the chart for adjectives. This is the definitive chart — we're missing just one more case, the vocative case, but for adjectives it's the same as the nominative. So, the chart:
case ma mi n f nom.sg. -, -i -, -i -œ -a acc.sg. -œg(a) -u dat.sg. -œm(u), -ome -oj gen.sg. -œg(a) -e ins.sg. -im -om dual (2-4) -a -e nom.pl. -i -a -e acc.pl. -e dat./ins.pl. -im gen.pl. -ih
We must also list forms of personal pronouns:
case 1st 2nd 3rd m 3rd n 3rd f nom.sg. ja ti on ono ona acc.sg. mene / me tebe / te njega / ga nju / ju, je gen.sg. nje / je dat.sg. meni / mi tebi / ti njemu / mu njoj / joj ins.sg. mnom tobom njim njom nom.pl. mi vi oni ona one acc./gen.pl. nas / nas vas / vas njih / ih dat./ins.pl. nama / nam vama / vam njima / im
There are no short (clitic) forms — instrumentals of personal pronouns can be anywhere in the sentence (clitic forms in plural apply only to dative).
Use Without Prepositions
Why is this case used for? It can be used with or without prepositions. Without prepositions, it means means, or medium of movement. For instance, with verb putujem, putovao "travel", and nouns šuma f "forest", cësta f "road" and vlak mi "train"; and using adjectives brz "fast" and velik "big", one can construct following sentences where all nouns are in the instrumental case standing for 'means' or 'medium' (you're traveling through...):
Putujem brzim vlakom. "I'm traveling by (a) fast train."
Putujem velikom šumom. "I'm traveling through (a) big forest."
Putujem brzom cëstom. "I'm traveling on (a) fast road."
Some more examples, illustrating 'tool-use':
Pišem olovkom. "I'm writing with (a) pen." olovka f "pen"
Radim čekićem. "I'm working with (a) hammer." čekić mi "hammer"
Slikam kistom. "I'm painting with (a) brush." kist mi "brush"
Although English distinguishes such 'tool-use' from 'means', Croatian does not.
We see that, without prepositions, it's used to state that one is using a train or road as means to travel, or some tools, but can also mean what one travels through. The last use can be stated also with the preposition kroz + noun in acc.:
Putujem kroz šumu "I'm traveling through forest"
Use With Preposition s(a) "with"
The instrumental is used also with prepositions, mainly with preposition s(a). It states 'company', and roughly corresponds to English "with":
Putujem s bratom. "I'm traveling with brother."
Igram se s mačkom. "I'm playing with (the) cat."
How is it supposed to be pronounced?! Well, together with the following word, so /sbratom/, /smačkom/, etc. Some people pronounce it separately, with a vowel similar to e in English "the". When the following word starts with an s-, z-, š-, ž-, or with 1st pers. mnom, the alternative form sa is used instead of s:
Jedem sendvič sa sirom. "I'm eating (a) sandwich with cheese."
Moj brat putuje sa mnom. "My brother is traveling with me."
Igram se sa sestrom. "I'm playing with (the) sister."
Some people use sa in all circumstances. It's discouraged in the Standard Croatian.
You will occasionally hear people use:
Putujem s vlakom.
Pišem s olovkom.
This is also considered not a good use. People understand it, and this is the norm in some dialects, but it's not considered good Croatian. (I will briefly cover dialects and regional variants later.)
So, an approximate rule would be: when in English you have "with blah blah" you should use in Croatian s(a) + instrumental of "blah blah"; but if it means the tool use ("writing with a pen") where you should use instrumental only.
Bear in mind that the instrumental is used with some other prepositions as well, and the preposition s(a) can be used with some other cases, having a completely different meaning! It's not that simple.
Updated 2014-11-06 (v. 0.4)